Method for drying wood



May 5, 1959 sAN Ac 2,884,707

METHOD FOR DRYING woon Filed Dec. 15, 1954 3 Sheets-Sheet 1 28 29 25Fig.1

May 5, 1959 JfsANbBAcK 2,884,707

METHOD FOR DRYING woon Filed Dec. 15, 1954 3 Sheets-Shet 2 May 5, 1959J. SANDBACK METHOD FOR DRYING WOOD 3 Sheets-Sheet 3 Filed Dec. 15, 19542 ,707. 1 7 METHOD FORDRYING WOOD j Jonas Sandbiick, LjusdaL Sweden IApplication' Decemb er ls, 1954, Serial No. 475,369 Claimspriority,application Sweden January 18,1954 '4 Claims. or 3436)' Theinvention relates to a method for drying wood.

, Itis known to dry wooclby means ofa mixture of air and combustiongasesgthe wood being placed in stacks in a. drying chamber, where thegas mixtureis introduced and whirled around by a fan arranged in thechamber. The present invention is based on .the principle of using sucha gas mixture as drying mediumandhas for its object an improved methodwhich permits of a more efiective utilization of the heat content of thegases at the same .time as the surface of the wood is preserved. Themethod comprises passing the wood and a mixture of air and combustiongases in countercurrent through a tunnel-shaped drying chamber, the gasmixture introduced into said chamber being held at'a temperaturebelow70.- C.

Stacked wood is fed through one end of the tunnel and is successivelydischargedat the other end, where the drying gas is preferablyintroduced. 'As mentioned, the temperature of the incoming gas must notexceed 70? C., as otherwise undesired reactions may. occur. rule itispossible to work with a temperature of 30-40 C. of the incoming dryinggas; lower temperatures cause a deteriorationof the dryingetfect. g p

The amount of drying gas and its speed of flow through the tunnel arepreferably so-adjusted that the temperature of the gas sinks to the dewpoint at the tunnel end 'where the wood last introduced is located.Thus, a condensation of steamon .the,cold Wood takes placeat this end,and the wood is warmed'by the heat of condensation freed. Tests haveshown that the woodmay even be heated to ,a temperature exceeding thegas temperature prevailing in the cold tunnel zone. I

In this way itis possible to effectively utilizethe, heat content of thedrying gas introduced. It is ideal if the gas escaping from the colderzone of the tunnel, while maintaining a good drying effect in.the'warmerzone, has about the same temperature, as'theisurroundingatmosphere. Provided that the drying tunnel is well heat insulated, theheatcontent of the combustion gases is .in such-case almost fullyutilized for the drying process,

Besides a good heat economy, however, the method accordingto theinvention has the further effect that condensible or soluble substancescontained in or added to the gas mixture are absorbed by the layer ofcondensation water formed on the surface of the still cold wood, whichfacilitates the penetration of said substances into the outer layer ofthe wood.

Of course, a great amount of carbon dioxide is always contained inthecombustion gases, and ifthefuel con sists of an oil having a normalsulphur content. also a certainamountof sulphur dioxide isobtained..f,.,IBoth these gases, particularly thesulphur'dioxide, aresoluble in water. The ,carbondioxide has .a suffocating effect on mostliving organismsand retards iidri-asiinilating fungi, while the sulphurdioxide acts disinfecting as well as bleaching. If the h'eatha's beengenerated bi "combustion of a fuel with little or'no sulphurco'ntentfsuch as wood, sulphur in some suitable rorrnmay be added to thefuel 6i introduced infotlifiame orabunier."

2,884,707 7 Patented May 5, 1959 As the=wood-is slowly moved towards thewarmerzone of-the-tunnel, -part of the absorbed carbon-dioxide and astances either -directly to the hot combustion gases-or to theless hotmixtureo f air and combustion-gases-which is-tobe introduced into thedryingtunnel. For killing insects and larvae for instanee-DDT-preparations may be added'tosaid gas-mixture.

'- A The wood introduced into the drying tunnel is preferably stacked inthe customary way, ie the layers of boards are laid on-crossers, so thatsufficient spaces are obtained for the passage of the drying gasin thelongitudinal d-irectionof thetunneh As t he wo od is usually advanced inits longitudinal direction thepassage area is furtherincreased, ifperforated'crossersare used.- In any'case, it lS-l essential that the"space between thesides of the wood stacks and the surrounding tunnel iskept as narrowas possible so thatthe greater part of the drying gas istowed to pass through the stacks.

Asa'

The method accordingto the invention will be described more closelybelow with reference to the accom panying drawings which show'diflerentembodiments of a dryingplant t Fig l shows adryi ngtunnel with a heatingaggregate in horizontal section.

Figs. 2 4 show in cross section different embodiments of drying tunnels.i

Fig. 5 shows, ln'horiz on tal section, a drying plant "with arrangementfor pre-heating the drying air.

Fig 6 shows a modification of the' plant in Fig.- 5.-

- Figs. 7 'and 8 show two different, modified embodiments of the dryingtunnel in Fig. l. Y

-Fig 9 shows; as seen from above, a view of a plant with arrangement forcirculation of'the drying gases.

Fig. 10 shows, as seen from above and partly in-horizontalsection,another plant with'gas circulation, and

Fig. 11 shows a vertical section along the line -Xl- XI in-Eig.-10; I fI The drying tunnel 12 shown in Fig.1 has preferably a rectangular crosssection and such a length that it can simultaneously receive a greatnumber of wood stacks l3' introduced after each other. The wood isfed'through the right end of the tunnel (in Fig. l) and is dischargedthrough-the left end. The two ends of the tunnel may be closed byfgates"or shutters 14, 15. M p I As mentioned above, the space between the woodstacks and-the surrounding walls of the tunnel must not have such awidth that the drying gas preferably seeks that way.- On the other hand,said space should he dimensioned to admit a sufficientamount of dryinggas to pass wood layers.

for drying the outermost portiorisof the wood. Therefore, it-issuitableto keep the space around the stacks of the same; agnitude'as thespace between the different -The-transporting means within the tunnelmaybe of many: difierentkinds, and Figs. 2-4 show only someerrrbodirnents. According to Fig. 2 the wood "stacks are advanced onrollers 16 rotatably journaled immediately above-thebottom of the tunnelcrosswise to its longitudinal' direction According to Fig. 3;th e woodisloaded on a wagon 17,- the wheels ofwhich runon :two rails 18. Toprevent an -=undesi-rablepassage of drying gasrunder the Wagonsthelatter are provided with screens 19 which wood is stacked on a suspendedwagon 20 carried by rollers 21 which run on rails along the sides of thetunnel immediately below the ceiling. Hinged screens 22 may be suspendedfrom the ceiling to prevent that too great an amount of drying gaspasses above the stacks.

An aggregate for producing drying gas is connected to the left end ofthe tunnel, 'i.e. the discharge end for the wood. A burner 23 for liquidor gaseous fuel, for instance fuel oil or generator gas, is arranged todirect a flame into a combustion chamber 24, the Walls of which arepreferably made of heat resistant brickwork and are well heat insulated.The burner may be of a usual type with or without a separate air fan.

The combustion chamber 24 has an air intake 25 provided with a filter ora fine meshed, net at its outer end toprevent the sucking-in of solidobjects. A damper 26 is provided for controlling the amount of air. Thecombustion chamber opens into a mixing chamber 27 having a larger airintake 28 with a filter 29 and a controlling damper 30.

The mixing chamber 27 is connected to the suction side of a fan 31 and'afurther filter 32 is inserted in the connection. The fan is driven by amotor 33 the speed of rotation of which may be controlled fordetermining the amount of drying gas to be introduced into the tunnel.Instead or besides, adjustable dampers may be arranged for the samepurpose. The outlet of the fan is connected to a pressure chamber 35through a fire shutter 34. The pressure chamber communicates with thetunnel 12 through a lattice-work 36 having fo'rits purpose to effect aneven distribution of the gas.

The plant is operated in the following way. As mentioned above, it ispresupposed that the wood stacks are positioned to leave free passagesfor the drying gas in the longitudinal direction of the tunnel. Thestacks are introduced through the right end of the tunnel (Fig. 1) andare advanced stepwise to the left, one stack length at a time, so that anew stack is introduced each time a dried stack is discharged throughthe left tunnel end. During the drying process the gates 15 at thedischarge end must be closed so that the drying gas is forced throughthe wood stacks. The flow resistance in the stacks is rather great and,therefore, the fan 31 should be capable of producing a pressure which,in dependence on the length of the tunnel and the free area of flow, mayamount to 50-100 mm. water column or more. At the intake end of thetunnel the gates 14 may be kept open to permit the gas to escape freely,although a separate gas outlet may be arranged. Drains may be providedin the bottom of the tunnel for removing the condense water formed inthe colder tunnel zone.

A pipe socket 37 is connected to the mixing chamber 27, and two otherpipe sockets 38, 39 open into the combustion chamber 24. These pipes areintended for addition of disinfecting, preserving and/or bleachingagents. The inlet 37 is preferably intended for highly volatilepreparations which evaporate at a moderate temperature, while the inlet38 is intended for less volatile preparations. The inlet 39 may forinstance be used for such additions as should undergo an oxidationprocess.

The damper in the air intake 25 to the combustion chamber 24 is adjustedto give the gas mixture leaving the combustion chamber the desiredtemperature. The temperature should preferably not exceed 400 C. Byadjusting the damper 30 fresh air is introduced into the mixing chamber27 in such an amount that the drying gas leaving said chamber is giventhe temperature most suitable for the drying in each separate case. Asmentioned, this temperature should not exceed 70 C. A heat responsivemember may be inserted in the pressure chamber 35 to close the fireshutter 34 by means of a servo-mechanism, in case the temperature shouldrise too high.

All adjusting operations required for obtaining a desired amount ofdrying gas at a desired temperature,

such as controlling the fuel supply to the burner, adjusting the dampersand the number of revolutions of the fan, may be performed automaticallyby regulators which are actuated by indicating members mounted atsuitable places. As such devices are known per se and do not constituteany part of the invention, they need not be more closely described.

In the embodiment shown in Fig. 5 the tunnel is divided into a dryingzone 40, to the right, and a cooling zone 41, to the left. The two zonesare separated by a mova'blescreen or curtain 42. The doors 15 at thedischarge end of the tunnel are kept open to let in fresh air whichpasses, in counterflow, the warm wood coming from the dryingzone,.whereby said wood is subjected to a secondary drying and coolingbefore being taken out of the tunnel. The air thus pre-heated is drawnoff at the inner end of the cooling zone through a tube 43 replacing theair intake 28 to the mixing chamber 27 in Fig. 1. Otherwise, the devicefor producing the drying gas mixture is exactly the same as the onedescribed above. The pressure chamber is connected to the drying zone ofthe tunnel immediately to the right of the screen 42. A considerablesaving of heat is thus achieved by supplying pre-heated air to themixing chamber.

Fig. 6 shows an embodiment difiering somewhat from that in Fig. 5. A fan44 is connected to the discharge end of the tunnel 12 to force fresh airthrough the cooling zone. The doors 15 should be kept closed. No screenis inserted between the two sections of the tunnel, but a tube 45connects the middle of the tunnel with a heating unit being constnlctedsubstantially as described above. The previous mixing chamber is herereplaced by a so-called compensating chamber 46 from which control theamount of cooling air supplied.

As the flow resistance may be rather great in long tunnels, it maysometimes be suitable to mount fans at intervals within the wholetunnel. Such an arrangement is shown in Fig. 7. The fan 49 is movableinto a pocket 50 at the side of the tunnel 51 so as to leave the passagefree when the stacks are to be advanced. Normally the pocket is closedby a hinged shutter 52. According to another embodiment, a fan 53 ismounted stationary in a similar pocket 54, and a screen 55 is insertedin the tunnel passage opposite to this pocket to force the air throughthe fan. Said screen 55 may be displaced through a corresponding slot inthe roof of the tunnel.

Fig. 8 shows another embodiment in which a fan 56 for supplying dryinggas is connected to the end of the tunnel 12 to blow in the longitudinaldirection. The tunnel has a side aperture for removing the dried woodstacks, said aperture being closed by a door 57 during the dryingprocess. This door may be pushed aside, as indicated by dash lines, andthe Wood stacks are discharged transversely by means of a wagon of somekind.

In large plants a number of separate drying tunnels may be connected toa common large aggregate producing drying gas. It is also possible toproduce only hot gases in a common aggregate, in which case each tunnelhas its own mixing chamber with fans for supplying drying gas having thedesired temperature. Alternatively, a large tunnel may be fed withdrying gas from several small aggregates. Of course, these arrangementsmay be varied to a great extent within the scope of the invention.

At certain occasions and weather conditions it may be advantageous touse a drying gas having a higher relative humidity than the one obtainedsolely by mixing with fresh air. One way of raising the humidity is toadd a spray of water which evaporates in the hot gas-air-mixture.Another way is to return a portion of the moist gas mixture escapingfrom the tunnel or to draw 01f a certain amount of drying gas at a pointalong the tunnel and to return that amount to the intake end. Thereby agood heat economy is obtained and any disinfecting or preservingsubstances contained in the drying gas are concentrated and more fullyutilized.

A plant for carrying out the modified method indicated above is shown inFig. 9. The plant comprises three parallel tunnels 12a, 12b, 120 havinga common hot-air aggregate, the fan 31 of which is connected to a drum58 divided into three branches, one leading to each tunnel. The hot-airaggregate and the fan are enclosed in a house 59. An underground channel60 communicating with the tunnels through apertures 61 in their floorsis provided under the opposite ends of the tunnels i.e. the intake endsfor the wood. One end of the channel 60 leads to a funnel 62 and itsother end opens into the house 59 where an under-pressure is prevailingowing to the suction of the fan 31. Thus, moist gases are sucked throughthe channel 60 into the house 59 to flow into the hot-air aggregatethrough the various intake openings thereof. The outlet opening 63 ofthe channel 60 is preferably directed towards the hot-air aggregate tocause the moist gas to hit the hot surface. directly so as to preventcondensation and decrease of the relative humidity of the gases. It issuitable to provide dampers in the inlet openings 61 of the channel forcontrolling the amount of circulating gases.

In the plant for circulating drying gas shown in Figs. and 11 it ispossible to keep different temperatures and moisture contents of thedrying gases introduced into the three tunnels 64a, 64b, 640. Above eachtunnel there is a channel 65 separated from the tunnel by means of aceiling 66. Apertures 68 controlled by dampers 67 are provided in saidceiling. A house 69 is divided into three difierent chambers 70 eachcommunicating with one of the channels 65 through openings controlled bydampers '71. Each chamber has an intake 72 for fresh air controlled by adamper. A fan 73 in each chamber 70 blows drying gases into the left endof the under-lying tunnel. A drum 74 leading from the hot-air aggregatein the house 59 has valve-controlled outlet openings 75 in the chambers70 in front of the three channels 65.

Owing to the suction of the fans 73 under-pressure is prevailing in thechannels 65. Therefore, if one of the dampers 67 is opened, drying gasfrom the underlying tunnel flows into the channel 65 towards its leftend, where it is mixed with hot air from the drum 74. The gas mixtureenters the chamber 70 connected to the channel in question and is therefurther mixed with air sucked from the outside. The proportions arecontrolled by means of the different dampers.

The humidity of the drying gas supplied by each fan 75 is controlled bymeans of the dampers 67. Thus, if only the damper located at the rightend of the tunnel is opened, the greatest addition of moisture isobtained. On the other hand, if a damper 67 at the middle of the tunnelis opened, the moisture of the returned drying gas is lower, and at thesame time a smaller amount of drying gas flows through the right sectionof the tunnel. In this way the drying effect may be controlled accordingto the requirements in each separate case.

What I claim is:

1. A method of drying wood, comprising passing the wood through atunnel-shaped drying chamber, introducing a mixture of air, combustiongases and sulphur dioxide into the tunnel at a temperature below C.,causing said gas mixture to pass in counterflow to the wood andadjusting the temperature and speed of flow of the gas mixture inresponse to variations of the dew point of the mixture at the tunnel endwhere the wood is introduced to obtain a condensation of steam on thewood last introduced into the tunnel, whereby the sulphur dioxidepresent will be dissolved in the condensate to form a disinfectingsolution on the surface of the wood.

2. A method of drying wood, comprising passing the wood through atunnel-shaped drying chamber, producing combustion gases containingsulphur dioxide by burning fuel oil containing sulphur and supplying airto said gases to get a gas mixture having a temperature below 70 C.,introducing said gas mixture into the tunnel and causing it to pass incounterflow to the wood, the temperature and speed of flow of the gasmixture being adjusted in response to variations of the dew point of themixture at the tunnel end where the wood is introduced to obtain acondensation of steam on the wood last introduced into the tunnel,whereby the sulphur dioxide present will be dissolved in the condensateto form a disinfecting solution on the surface of the Wood.

3. A method of drying wood, comprising passing the wood through atunnel-shaped drying chamber, introducing into the tunnel at a pointremote from its ends a mixture of air, combustion gases and sulphurdioxide having a temperature below 70 C., causing said gas mixture topass in counterflow to the wood, introducing fresh air into the tunnelend, where the wood is discharged, and causing it to pass in counterfiowto the dried wood whereby it is preheated, withdrawing this preheatedair from the tunnel and supplying it to combustion gases to obtain thegas mixture to be introduced into the tunnel, and adjusting thetemperature and speed of flow of the gas mixture in the tunnel inresponse to variations of the dew point of the mixture at the tunnel endwhere the wood is introduced to obtain a condensation of steam on thewood last introduced, whereby the sulphur dioxide present will bedissolved in the condensate to form a disinfecting solution on thesurface of the wood.

4. A method of drying wood, comprising passing the wood through atunnel-shaped drying chamber, introducing into the tunnel a mixture ofair, combustion gases and sulphur dioxide having a temperature below 70C., causing said gas mixture to pass in counterflew to the wood, wherebyit is preheated, withdrawing moist gases from some point along thetunnel and supplying them to the gas mixture to be introduced into thetunnel, and adjusting the temperature and speed of flow of the gasmixture in the tunnel in response to variations of the dew point of themixture at the tunnel end where the wood is introduced, to obtain acondensation of steam on the wood last introduced, whereby the sulphurdioxide present will be dissolved in the condensate to form adisinfecting solution on the surface of the wood.

References Cited in the file of this patent UNITED STATES PATENTS Re.8,240 Johnson May 21, 1878 105,720 Parrott July 26, 1870 170,928 AndrewsDec. 7, 1875 1,672,326 Kobiolke June 5, 1928 2,040,884 Somes May 19,1936 2,706,344 Vaughan Apr. 19, 1955 2,707,695 Courtier May 3, 1955

